1. Field of the Invention
The invention relates to acrylate copolymers having pressure-sensitive adhesion properties and to pressure-sensitive adhesives prepared on the basis of such acrylate copolymers. These acrylate copolymers and pressure-sensitive adhesives are especially suitable for bonding low-energy surfaces.
2. Description of the Related Art
A disadvantage of known, commercially available pressure-sensitive adhesives based on acrylate copolymers is their inadequate adhesion to substrate surfaces having a low surface energy. Critical, low-energy surfaces of this kind are encountered on numerous articles of everyday life, and also on construction elements or assembly components, for example in automotive engineering, in the furniture industry and the construction industry. Materials characterized by low-energy, apolar surfaces include not only polypropylene and polyethylene but also, in particular, ethylene-propylene-diene copolymers (EPDM), powder coatings, silicones, fluorocarbon-modified surfaces, and polytetrafluoroethylene (Teflon®).
Adhesives known to achieve effective adhesion even on low-energy surfaces include on the one hand pressure-sensitive adhesives based on natural rubber or synthetic rubber and on the other hand pressure-sensitive adhesives based on polysiloxanes. The possibilities for use of the pressure-sensitive adhesives, however, are very limited. Rubber-based pressure-sensitive adhesives are sensitive to exposure to oxygen, ozone and light owing to the presence of C═C double bonds. This results in a lack of (adequate) ageing stability. The use of pressure-sensitive polysiloxane adhesives is inappropriate for many applications owing to the high price.
It is known that the adhesion of pressure-sensitive polyacrylate adhesives to low-energy surfaces can be enhanced by adding tackifiers, such as tackifying resins, and/or plasticizers. However, additions of this kind have the drawback of an unfavourable effect on cohesion, ageing stability and temperature stability.
The patent literature has described a number of pressure-sensitive adhesives (PSAs) based on acrylate copolymers, aimed at enhancing the adhesion to low-energy surfaces.
U.S. Pat. No. 4,243,789 names N-vinylpyrrolidone and hydroxyl-containing monomers as adhesion-enhancing.
U.S. Pat. No. 4,726,982 describes PSAs based on crosslinked acrylate copolymers, synthesized from acrylic ester monomers and N-vinyllactam monomers. Additionally present are tackifying resins, such as poly(isobornyl methacrylate), for example.
U.S. Pat. No. 4,728,684 discloses acrylate-modified polyester urethanes and chlorinated rubbers which are crosslinked by isocyanate. This is said to enhance the adhesion to low-energy surfaces, e.g. PVC films.
U.S. Pat. No. 4,994,538 relates to PSAs which are prepared by emulsion polymerization and contain reactive silicone acrylate polymers in order to enhance the adhesion properties of these emulsion polymers.
U.S. Pat. Nos. 5,874,143 and 6,280,557 describe acrylate copolymer-based PSAs whose adhesion to low-energy surfaces is achieved through modification with resins and plasticizers.
U.S. Pat. No. 5,840,783 describes PSA compositions which comprise chlorinated polyolefins and exhibit enhanced adhesion to low-energy surfaces of plastics.
U.S. Pat. No. 5,434,213 describes copolymers of ethylene-vinyl acetate with acrylate monomers for use in PSAs which are suitable for the bonding of low-energy surfaces.
U.S. Pat. No. 5,602,221 relates to PSAs exhibiting enhanced adhesion to low-energy surfaces; these are copolymers of acrylic ester with a mixture of polar and apolar, ethylenically unsaturated monomers, the homopolymers of these monomers having different glass transition temperatures.
U.S. Pat. No. 5,817,426 discloses PSAs for low-energy surfaces, based on acrylate copolymers prepared by emulsion polymerization. The monomer mixture contains from 20 to 30% by weight of isobornyl acrylate, along with alkyl acrylate monomers. It is assumed that the isobornyl group influences the adhesion and rheology properties of the PSA; furthermore, it is mentioned that isobornyl acrylate homopolymers have a high glass transition temperature.
In practice it has been found to be the case that the increase in glass transition temperature (Tg) entailed for the use of isobornyl (meth)acrylate makes the adhesive product so hard that an adequate tack is achievable only by reducing the molecular weight. This molecular weight reduction, however, leads at the same time to a decrease in the shear strength and in the temperature stability.
None of the inventions described in the abovementioned documents, however, has led to an adhesive product which without the use of tackifiers and/or plasticizers achieves sufficient adhesion to the abovementioned critical, low-energy surfaces. As already mentioned, the use of such substances is associated with adverse effects.